This invention relates generally to a coating for improving the thermal emittance of an x-ray tube anode and more particularly to the application of said type coating to the anode target region employed in said x-ray tube.
X-ray tubes accelerate a beam of electrons through a vacuum to high electron velocity under a high electric field toward a metallic target. When the electrons are decelerated by impact with the target, a beam of x-rays is emitted by the target. Only about one percent of the electron energy produces x-rays and the remainder is dissipated as heat. It is customary to aid this dissipation by applying a thermal emittance coating to the target.
A known emissive coating for said purpose is disclosed in U.S. Pat. No. 4,132,916, which is assigned to the assignee of the present invention, and comprises up to 20 percent by weight of a high thermalemittance material TiO.sub.2 with the remainder of the composition being made up of an oxide for raising the melting point to an acceptable level and a small amount of a stablizing material for stability of the oxide over the device operating temperature range. This coating is disclosed to be made by sintering a mixture of calcium oxide, zirconium oxide and titanium dioxide to form a ceramic mass. The ceramic mass is ground and and screened for a suitable range of particle sizes, such as, for example, from about 10 to 37 microns. The powder is aplied to the target by conventional plasma spray techniques. Finally, the anode target, including the powder coating, is heated to sufficiently elevated temperatures to fuse the powder to the surface and to out gas the target. Said prior art emissive coating powder requires a firing temperature of about 1640.degree. C. to produce a smooth adherent coating. Modern x-ray targets conventionally employ molybdenum alloys or tungsten alloys which can liberate carbon impurities at temperatures exceeding about 1600.degree. C. Such impurities can thereafter react with the emissive coating at the interface to produce carbon dioxide gas which disrupts adhesion of said coating. The deposition surface of such conventional refractory metal x-ray targets is also generally prepared by sandblasting with Al.sub.2 O.sub.3 grit to facilitate subsequent adhesion of the final emissive coating. Residual impurities from such sandblasting material can thereby further contaminate the deposited coating and lower its melting point which is understandably objectionable.
In a more recently issued U.S. Pat. No. 4,600,659, also assigned to the present assignee, there is disclosed a related ceramic coating to enhance the thermal conductance of such x-ray tube targets. Said ceramic coating comprises a fused material containing from about 40 percent to 70percent TiO.sub.2 with the remaining material being a stabilizing oxide selected from the group of CaO and Y.sub.2 O.sub.3. Again, the difficulties being experienced with the earlier discovered emissive coatings are not thereby avoided since the melting point of said coating can still be reduced by the same impurities to below 1400.degree. C. A preferred embodiment for said type emissive coating describes the stabilized oxide material as comprising 92 weight percent ZrO.sub.2 and 8 weight percent CaO. While such coatings from the ternary TiO.sub.2 --CaO--ZrO.sub.2 system exhibit emissivity values of over 0.800 along with good adhesion at room temperature, they melt or peel off of the target at operating temperatures of modern x-ray targets which can reach 1400.degree. C. and higher. The present invention represents an improvement over these prior art emissive coatings in all said regards, although both above mentioned patents are specifically incorporated herein by reference due to preparation for the present coatings being essentially the same as therein disclosed.
As also disclosed in the above referenced U.S. Pat. No. 4,132,916, the thermal energy being generated by a rotating anode x-ray tube during operation is dissipated primarily by radiation from the target to a surrounding fluid-cooled casing. The basic construction for a conventional rotating anode x-ray tube device comprises a sealed evacuated glass envelope incorporating cathode and anode structural assemblies to generate X radiation within said glass envelope, said cathode structural assembly including an electron emissive filament operatively associated with means to focus an electron beam generated by said filament upon the anode structural assembly, said anode structural assembly including a refractory metal target for impingement of said electron beam thereon to produce X radiation and further structural means being disposed with said glass envelope to cause relative rotation between said cathode assembly and refractory metal target. As further disclosed in said referenced patent, the cathode assembly remains stationary during tube operation while the anode assembly rotates with respect thereto. Since convection cooling from said high vacuum tube is not possible, however, a great amount of heat must be radiated through the glass envelope and hence to the oil circulating in the tube casing. Representative x-ray tubes of said type are now being sold by General Electric Company under the product designations: CT9000, CT9800, DR1190B/BR, and D1191B/BR. The CT designation for said products makes reference to the utility of said rotating anode x-ray tubes for various computerized tomography applications. Accordingly, it has now been found that the presently improved thermal emittance coating is particularly suitable for use in such type radiographic equipment.
One such already known type radiographic imaging system which can be improved according to the present invention utilizes an x-ray source, which frequently comprises a rotating anode x-ray tube, a ceramic scintillator body to convert the x-rays to an optical image, and photodetection means coupled thereto for converting said optical image to an electronic display thereof. Said radiographic imaging system can further include means for digital recording of said optical image to include digital processing means to enhance the quality of said optical image. Computerized tomography imaging systems of said type are disclosed in U.S. Pat. Nos. 4,242,221 and 4,421,671, also assigned to the present assignee, along with suitable ceramic scintillator materials. In the operation of said type equipment, the X radiation emerging from said rorating anode x-ray tube is frequently columnated to produce a thin beam of x-rays which are then projected toward moving x-ray detector means. A subject or body to be examined is positioned in the path of the x-ray fan beam in such a manner that the beam is attentuated as it passes through said subject with the amount of attenuation being dependent upon the density of the said subject. The moving radiation detector means frequently comprises a detector array having a plurality of channels defined therein with said channels being structurally configured so as to receive the attenuated fan beam of x-rays to produce electrical signals which are dependent upon the radiation received within each channel. The electrical signal readings emerging from said channels at a plurality of angular positions with respect to the subject being examined while the x-ray source and detector means are rotated about said subject can therefore be digitized and transmitted to computer means which uses one of a number of available algorithms to compute and construct a picture of the cross section traversed by the fan beam of x-rays. The resulting picture can thereafter be displayed on a cathode ray tube or, alternately, may be used to create an image on permanent media such as photographic film or the like. Accordingly, it follows that the presently improved thermal emittance coatings can be incorporated into said type radiographic imaging equipment as a means for achieving improved operation.
It is a principal object of the present invention, therefore, to provide an improved thermal emittance coating for an x-ray tube anode target which exhibits high thermal emittance along with improved adhesion to said target during tube operation.
It is still another important object of the invention to provide a ceramic coating for an x-ray tube anode which can be deposited on the variety of refractory metal targets as a blended metal oxide mixture for simpler material handling and processing costs.
Still another important object of the present invention is to provide an improved thermal emittance coating not as subject to the impurity contamination problems ordinarily encountered during processing of this coating on a fractory metal anode target.
It is a still further object of the present invention to provide a method for coating an x-ray tube anode target with a more adherent coating of improved emissivity.
A still further object of the present invention is to provide x-ray tube and x-ray imaging devices exhibiting improved performance attributable to the present ceramic coating.
These objects and other features and advantages for the present invention will become more readily apparent upon reference to the following description when taken in connection with the accompanying drawings.